Richie's attorney, Shawn Chapman Holley, said in court that Dr. Terence Sean McGee will testify about tests and conclusions by drug experts used by prosecutors.
Holley told Superior Court Commissioner Steven K. Lubell that McGee's testimony was critical to the defense. But he is not available to appear in court until next month, the lawyer said.
Richie, 25, was not in court for the brief hearing. Because she faces a misdemeanor charge, the co-star of TV's "The Simple Life" was not required to attend.
Terms of the deal were not made available and the district attorney's office declined to comment.
Richie was arrested early on Dec. 11 after witnesses reported seeing her black 2005 Mercedes-Benz sport utility vehicle headed the wrong way on a freeway in Burbank.
She allegedly failed a field sobriety test and authorities said she told them she had smoked marijuana and taken a prescription painkiller. No drugs were found on her or in the vehicle.
She pleaded not guilty in February to the DUI charge.